better support for view types (#15436)

* you can put borrows into tables

* enforces mutating views only mutate mutable data

tests/views/tdont_mutate.nim

@@ -0,0 +1,48 @@
+discard """
+  cmd: "nim check --hints:off $file"
+"""
+
+import tables
+
+{.experimental: "views".}
+
+const
+  Whitespace = {' ', '\t', '\n', '\r'}
+
+proc split*(s: string, seps: set[char] = Whitespace,
+                maxsplit: int = -1): Table[int, openArray[char]] =
+  var last = 0
+  var splits = maxsplit
+  result = initTable[int, openArray[char]]()
+
+  while last <= len(s):
+    var first = last
+    while last < len(s) and s[last] notin seps:
+      inc(last)
+    if splits == 0: last = len(s)
+    result[first] = toOpenArray(s, first, last-1)
+
+    result[first][0] = 'c' #[tt.Error
+      attempt to mutate a borrowed location from an immutable view
+    ]#
+
+    if splits == 0: break
+    dec(splits)
+    inc(last)
+
+proc `$`(x: openArray[char]): string =
+  result = newString(x.len)
+  for i in 0..<x.len: result[i] = x[i]
+
+proc otherTest(x: int) =
+  var y: var int = x #[tt.Error
+    'y' borrows from the immutable location 'x' and attempts to mutate it
+  ]#
+  y = 3
+
+proc main() =
+  let words = split("asdf 231")
+  for i, x in words:
+    echo i, ": ", x
+
+main()